Bill over at fertanish.net has discovered his homemade 10 gallon terrarium is certainly harboring some stow-away critters that hitched rides on his initial soil and plant supply. While oft times these unexpected terrarium inhabitants are unwanted - I agree with Bill that they can also provide a neat little opportunity to watch the critters in action.
I collected soil from the back yard; I'm sure I captured a number of slug eggs as they are appearing everywhere. I've also found at least one Sow Bug, a couple of earth worms, possibly two centipedes, and a couple of spiders. Amusingly, the millipedes have mostly dug under for the winter; I now barely see them at all.
I focused building the terrarium for insects rather than plants, but certainly didn't consider starting an ecosystem. It's very entertaining.